How much vitamin D do I need?

Consumer surveys find that vitamin D now outsells all other supplements, including fish oil, probiotics and even multivitamins. That’s rather surprising given that vitamin D is actually one of the few vitamins we can make on our own. All we need is to expose our skin to the sun, and our bodies take care of the rest. But the D boom isn’t a scam, necessarily – it’s just rife with confusing science and misinformation.

Vitamin D is essential for healthy bones: We need it to absorb calcium, among other minerals. It is also being investigated as a possible preventive player in conditions including asthma, cardiovascular disease, hypertension, autoimmune diseases like Type 1 diabetes and, of course, cancer. Although none of these associations has been proved – a fact that is often overlooked – breathless headlines provide enough intrigue to drive market growth above 10 percent a year.

The other factor driving growth is that many of us don’t get direct exposure to the sun – either because we wear sunblock or because we’re indoors all day. And with the exception of fish liver, very few foods are naturally rich in vitamin D. Instead, the vitamin D in most foods and supplements is created by irradiating lanolin (the greasy substance in sheep’s wool).

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If you don’t eat fortified foods, you’re probably not getting much vitamin D through your diet.

Here’s the catch: It’s not clear how much is enough.

The Institute of Medicine, the organization that sets recommended dietary allowances for the general U.S. population, settled on 600 IU per day for healthy adults who get minimal sun exposure.

Many vitamin D experts, however, say that recommendation is too low because it’s based only on the amount of vitamin D that’s necessary for bone health and calcium metabolism. (The Institute of Medicine examined the vitamin’s other effects, but determined that the evidence wasn’t yet strong enough to support a higher recommendation.) These critics argue that to capitalize on vitamin D’s other potential benefits, we need more.

This disagreement means that vitamin D blood tests are more like a Rorschach test than an actual diagnostic tool: Whether you’re “deficient” depends largely on your doctor’s interpretation of what constitutes “adequate.” What’s more, the same blood sample can produce different results depending on which lab does the processing.

Despite all this uncertainty, it’s still possible to make an intelligent decision about whether to take a vitamin D supplement. You just need to ask yourself some questions:

Do you spend 15 minutes or more in the sun without sunscreen or protective clothing several times a week? If so, you’re likely to be in good shape in terms of vitamin D for most of the year. (Our bodies know when to stop making vitamin D, so there’s no risk of overdose from the sun.)

Do you live north of the line that would connect Philadelphia to San Francisco? If so, the angle of the sun will make it extremely difficult to make adequate vitamin D from the sun, especially in winter. Indoor jobs, constant sunscreen use, and/or protective clothing will also get in the way.

Are you breast-feeding? If so, you likely will want to give supplemental vitamin D to your baby. There is very little vitamin D in breast milk, further evidence that we have evolved to meet our requirements with the help of the sun, not food.

If you insist on absolute scientific certainty, you can stick to the official RDA of 600 IU for bone health, keeping in mind that RDAs are guidelines for the general population’s average consumption over time. If you want to hedge your bets in case our bodies need more vitamin D for optimal function, there’s nearly no risk associated with consuming up to 4,000 IU per day.

Many experts argue that 10,000 IU or more is probably safe for most people. But don’t go crazy: There’s no consensus that super doses are good for you, and over the long term, too much vitamin D can cause dangerously high levels of blood calcium, among other issues, which can damage the heart, blood vessels and kidneys.

Human nutrition is full of mysteries, and vitamin D is one we’re not likely to solve any time soon. But that shouldn’t prevent us from making informed decisions based on what is understood about the vitamin – and what we know about ourselves.